As more and more segments of the business environment enter the information age, more and more computers and increased computing power are required. As businesses move from the old to the new economy, their reliance on the processing, transference, and storage of digital information is becoming a more critical aspect of their overall business strategy. While in the past computer crashes were seen a mere nuisance, the loss of computing power and business data may well devastate a businesses' ability to survive in today's new economy. As such, the need for reliable, uninterruptible electric power to maintain the operational status of the computing equipment and the integrity of the digital data continues to rise.
To meet these requirements, uninterruptible power supplies (UPS) have been developed. These UPS's utilize a bank of electric storage batteries and solid state conversion equipment in association with the utility line voltage to provide continuous electric power to a businesses computer system in the event of a loss or deviation of power quality from the utility. The number of batteries contained within a UPS is dependent upon the business' length of time that it needs to operate in the event of a utility power system failure. Likewise, the number of power modules included in a modular UPS, or the power rating of a power conversion module in a fixed-size UPS, is dependent on the overall total system load required to be supplied thereby. As the electrical utilization requirements of a business' computing system grow, additional power modules or additional UPS's may be purchased and integrated into an overall uninterruptible power system for the business enterprise.
While a UPS is required to supply the entire electrical requirements of a system to which it is applied during loss of utility power, and while a business may choose to operate its UPS to condition the utility line power to provide high power quality to their computing equipment, during periods of normal utility line availability operation of the UPS may provide more inefficiencies than advantages. However, since the loss or corruption of utility line voltage often may not be predicted, disconnection of the UPS may result in momentary loss of utility power to the computing equipment and corresponding, loss of computing power and electronic data.
To overcome this problem and to increase the efficiency of the UPS during periods of normal utility line voltage operation, typical UPS's include some form of bypass circuitry to route the utility line voltage directly to the UPS output to which the computing equipment is coupled. Such a configuration of a typical UPS is illustrated in FIG. 26. As may be seen from this simplified single-line schematic, the AC line voltage input 101 is routed through a bypass circuit 103 to the UPS output 105 coupled to the load 107. By utilizing this bypass circuit 103 losses resulting from rectification of the AC line input voltage as well as losses resulting from the generation of an AC output voltage waveform through switches 111, 113, 115, and 117 may be avoided. Typically this bypass circuitry 103 comprises a back-to-back silicon controlled rectifier (SCR) circuit, although other bypass circuitry configurations are also applicable. Unfortunately, the addition of the bypass circuitry 103 adds substantial cost, thermal management problems, and volume to the UPS itself. Such disadvantages have long been accepted as a necessary evil to allow high efficiency operation during periods of normal utility line voltage operation.
Transition from this high efficiency bypass mode of operation to inverter operation requires that the inverter's bus capacitors 119, 121 be charged. Some prior UPS systems utilize soft charging circuitry comprising additional power semiconductor devices per capacitor or per bus (not shown) to control the charge rate. Alternatively, the front end devices 111, 113 could be modulated to bring the capacitors up to the proper voltage to allow proper output waveform generation. Unfortunately, these prior UPS systems were unable to supply power during this soft charging period. As a result, a momentary loss of the output voltage waveform could be experienced until the capacitors 119, 121 reach their proper charge. Additionally, the required additional power devices and requisite circuitry adds costs and thermal management problems as well as volume to the UPS system, which disadvantages have heretofore merely been accepted.
To allow scalability of the power provided by a UPS, each UPS power module must be able to generate an output voltage waveform in coordination with the other UPS power modules to supply the entire connected load. In addition to providing scalability, this configuration provides redundant operation to maximize the fault tolerance of the UPS and ensure continued electrical supply to the computing equipment. To ensure that a failure within any one of the power modules of the UPS does not result in the entire UPS being taken off-line thereby resulting in a loss of electrical supply to the computer equipment, each individual power module of a typical UPS system includes in-line fault isolation circuitry 123 that operates to isolate a failed power module from the output 105. Typically this fault isolation circuitry takes the form of in-line power semiconductors, back-to-back SCR's, electromechanical relays, etc. Unfortunately, this fault isolation circuitry 123 adds costs, thermal management problems, and volume to the UPS system.